Booms for loading and unloading materials, whether on land or ship decks, are known, and typically are secured to a fixed supporting point and rotatable around at least a part of a circle.
Such booms when intended for use secured to the deck of a ship, for use in unloading materials contained on the ship, are typically rotated (slewed) around a point on the deck. This rotating motion of the boom has been traditionally achieved by slewing actuators, with the boom structure typically being connected to the slewing actuator by trunnion pins. Typically, such a structure will slew from 90 degrees to 120 degrees in either direction to discharge material to another ship or on shore. Such range of motion generally relates to restrictions based on the space available on the ship deck, and not by any operational limits. Similar ground-surface based arrangements are also used for cargo movement.
A conventional slewing actuator typically comprises a hydraulically actuated rack and pinion arrangement. In such an arrangement, hydraulically actuated steel racks move back and forth to effect rotation about the rotatable pinion, which cause the boom to move. The total length of the rack governs the amount of rotation and, typically, these arrangements can take up a large amount of space in order to operate. On some ships, and other loading and unloading locations, there may be a small clearance envelope available in which such an arrangement can be positioned, and this can limit the rotational range of the boom to, for example, 90 degrees in some situations, which may not be sufficient for unloading operations.
There are other problems inherent in such slewing actuator arrangements. Firstly, slewing actuators are typically manufactured and shipped as integral units, and are not disassembled for shipping from the manufacturing facility to the shipyard or other intended use location, thus making transport by air freight or other standard means expensive and difficult, as the equipment used in these systems is very large, heavy and expensive.
Secondly, such systems can be very difficult to install, particularly when intended for use on ships, as they are typically affixed to the ship deck by kingpin bushing arrangements mounted to the ship deck, in which a vertical pin is positioned through a key opening, to the hull. This requires cooperation between the key opening and the kingpin so that a proper fit is ensured. However, achieving such compatibility can be difficult since these are each usually fabricated by different manufacturers to very tight tolerances.
Thirdly, conventional slewing actuators have a limited number of specific sizes available, often leading to extremely large, over-designed actuators, when the ideal size would have been in between two available sizes.
It would therefore be advantageous to have a slewing actuator system suitable for use on ships, land surfaces or docks, and possessing a more compact design that allows for a greater amount of rotational range of the boom attached thereto, particularly on ships or at other use locations having a small clearance envelope.
It would be further advantageous to have a slewing actuator system which is easy to install, lighter than conventional slewing actuator systems, and which can be easily disassembled into separate portions for shipping, if necessary.
It would be still further advantageous to have a boom slewing actuator system which safely prevents boom slippage which securely locks the boom in place when hydraulic pressure is removed.